Crystal furnaces for the growth of single-crystal ingots by the Czochralski method typically include a crucible contained within a sealed vacuum or an inert environment to contain a melt and a seed lift and rotation mechanism in a pulling head for drawing the crystal from the melt along a vertical axis at a steady rate while providing relative motion about this vertical axis with respect to the melt. Crystal growing furnaces employ a pull or pulling head in order to provide for a seed lift and a seed rotation mechanism and to maintain the mechanical alignment of the axis of rotation of the seed with the vertical pulling axis of the seed. The single ingot crystals are grown from a seed crystal, which seed crystal is held in a chuck or other means so that the seed crystal may be lifted and rotated. The means for lifting and rotating the seed crystals is frequently a flexible, elongated member, such as a cable or bead chain, which cable or chain is raised by a winch mechanism. The winch is mounted on a rotatable platform mounted on a platform on the upper portion of the crystal growing furnace. The entire winch mechanism to which the cable or chain is secured is secured to the platform for rotation therewith, so as to provide for rotation of the cable or chain, so that at the same time the seed, chuck, and crystal are made to rotate the winch is lifting the seed crystal from the melt enclosed in a sealed vacuum or inert atmosphere.
Generally, both high and low speed seed lift motors are employed on the rotatable platform with the seed lift motors usually having a respective defined high and a low speed range. The motors are mechanically combined on the platform by using differential or clutch means, so as to combine the motion of the two motors on the platform. The seed rotation motor is usually not mounted on the platform, but is mechanically coupled to the platform through a drive belt to rotate the platform. In order to control adequately the crystal growing process both the seed lift and the rotation rate must be independently controllable. In addition, it is convenient to provide two or more separate speed ranges for the seed lift motion. Thus, the seed lift and rotation mechanism for crystal growing furnaces comprises both a high speed and a low speed lift motor and a rotation motor all mechanically combined to a winch which must rotate about its own axis to raise the seed crystal and which also may translate along its axis, such as by a spline and lead screw at the time of winch rotation to maintain the cable or chain on the central vertical pulling axis. The crystal growing furnace pulling head for use with a flexible chain is described, for example, in U.S. Pat. Nos. 4,301,120 and 4,371,502.
At present, the pulling heads for crystal growing furnaces are quite complex and cumbersome and involve at least two motors with the associated electric wiring controls, a winch, a cable or a chain all mounted on a rotatable platform which makes adjustment, alignment, and maintenance of the pulling head mechanism quite complex and difficult. In addition, the heavy mass on the rotatable platform requires that the platform be carefully balanced and constructed to support the weight, while there also is a requirement for electrical slip rings in order to provide electrical power to the motors and electrical sensing of speed- and position-transducer signals on the platform while the platform is rotating. Accordingly, there is a need for a simplified, less complex, easily maintained, and more effective pulling head for use with crystal growing furnaces.